A key component in any wireless communication system is an RF power amplifier circuit that is enabled by a wide variety of semiconductor technologies. These RF power amplifier circuits generally operate in a variety of modes and hence the term ‘Multi-mode RF power amplifiers’. The multi-mode RF power amplifier circuits used in 3G/4G mobile handsets utilize switch path architecture. According to the architecture, switches placed in various paths, turn on or off to respectively activate or deactivate a particular path. A conventional multi-mode RF power amplifier circuit is designed to operate under two power modes, a high power mode and a low power mode. The conventional multi-mode RF power amplifier circuit includes a high power path and a low power path that are placed parallel to each other. In the high power mode, the high power path is active and the low power path is inactive. Whereas, in the low power mode, the low power path is active and the high power path is inactive. The RF power amplifier circuits are designed in such a way that only one of the two paths is active at any given time. This is achieved with the help of switches placed in these paths. Switches, such as FET switches, placed in the high and low power paths control the switching between the two parallel paths. The voltage difference between a gate and a drain of the FET switch determines the switching on and off of the FET switch. The FET switch will be switched on when the gate and drain terminals are biased with a same voltage and switched off when the gate and drain terminals have a potential difference that is less than a pinch-off voltage of the FET switch. The logic levels available for biasing the FET switch are 0V and the battery/power amplifier collector bias voltage.
In the prior art, under the high power mode of operation, when a supply voltage Vcc is reduced to a certain level such that the voltage difference between the gate and drain terminals of the FET switch placed in the inactive low power path becomes higher than the pinch-off voltage of the FET switch, the FET switch placed in the inactive low power path is switched on. Consequently, a portion of output power from a power amplifier placed in the high power path is diverted into a matching network in the low power path rather than being transferred to an RF output terminal. Therefore, the power amplifier placed in the high power path has to deliver higher output power to compensate for the loss of output power to the low power path when the supply voltage Vcc is reduced. This results in a considerable amount of performance degradation in terms of gain, current consumption and linearity.
In view of the foregoing, there arises a need for an efficient multi-mode RF power amplifier circuit that provides improved isolation between the active and the inactive power paths.